Electronic security systems are known for the detection of the unauthorized removal of items containing a resonant tag circuit. A preferred system is described in U.S. Pat. Nos. 3,810,147, 3,863,244 and 3,967,161. Such systems employ a transmitter providing an electromagnetic field in a zone or region under surveillance, and through which items must pass for detection, and a receiver operative to detect the field disturbance caused by the presence of a resonant tag in the surveillance zone and to provide an output alarm indication of tag presence. In these electronic security systems described in the aforesaid patents, two identical planar loop antennas are usually employed, one for transmitting and one for receiving. The transmitting loop antenna generates an electromagnetic field which is repetitively swept through a predetermined frequency band which includes the resonant frequency of the tag circuit. The receiving antenna is operative to sample the field generated by the transmitter and to detect the change in this field caused by the resonant circuit.
An antenna system is described in copending applications Ser. No. 878,753 filed Feb. 17, 1978, and Ser. No. 92,325 filed Nov. 8, 1979 (GL-12A), of the same inventor as herein, which provide improved performance in the associated electronic security system in reducing high intensity fields at distances outside of the interrogation region and in reducing the sensitivity to interferring signals originating outside of the interrogation region. The antenna system of copending application Ser. No. 878,753 comprises a pair of substantially identical planar loop antennas respectively connected to the transmitter and receiver of the security system and providing an electromagnetic field of high intensity in the interrogation region of the system, while preventing high intensity fields at distances outside of the interrogation region which are large in comparison to the antenna dimensions. The antenna system also discriminates against interferring signals originating outside of the interrogation region at distances large compared with the antenna dimensions.
Each planar antenna includes two or more loops lying in a common plane, with each loop being twisted 180.degree. with respect to each adjacent loop to be in phase opposition. The transmitting antenna and receiving antenna are symmetrical, that is, identical or nearly so with respect to the number and size of the two or more loops, and are cooperative in that twisted loops of the receiving antenna reverse or decode the adjacent phase relationship of the twisted loops of the transmitting antenna. For each antenna, the total loop area of one phase is equal to the total loop area of opposite phase in order to achieve optimum performance.
The antenna system of copending application Ser. No. 92,325 is similar, but the two cooperating planar antennas are not symmetrical to each other. The transmitting antenna can be a single loop planar antenna while the receiving antenna can include two or more loops lying in a common plane with each loop being twisted 180.degree. with respect to each adjacent loop. Alternatively, the transmitting antenna can have two planar loops and the receiving antenna three planar loops, the loops of each antenna lying in a common plane with each loop being twisted 180.degree. with respect to each adjacent loop to be in phase opposition.
The antenna system of both copending applications are effective to reject radio frequency interference generated by magnetic fields at distances from the antenna large compared to the antenna dimensions. However, such antennas are still susceptible to electrical noise which is coupled capacitively to the antenna. Such capacitive noise coupling is illustrated in FIG. 8 whereas the noise is represented by a source 90 which is capacitively coupled to the antenna. The differential amplifier 92 represents a typical front end circuit of the system receiver.
Referring to FIG. 8, magnetic fields generated at a distance large compared to the dimensions of the antenna couple equally to loop #1 and loop #2. Since these loops are twisted 180.degree. with respect to each other and are equal in area, the net voltage of the two loops cancel each other. Noise capacitively coupled to one side of the antenna, however, is not cancelled out. As illustrated, the noise source 90 is capacitively coupled to the lower loop (#2), and strongly coupled to only one side of this loop. The signal path "A" from the noise source to a differential amplifier 92 is much longer than the path "B". Therefore, the impedance of path "A" is much greater than path "B". As a result, the noise signal capacitively coupled to the antenna produces a real signal at the positive input of the differential amplifier.
It is the object of the present invention to provide an antenna system for use in a resonant tag detection system which is not sensitive to capacitively coupled noise, as well as offering the advantages of electromagnetic noise rejection and other benefits offered by the antenna system of the aforesaid copending applications.